The main obstacle to the widespread utilization of solar cells is the high cost of solar cell production, with typically almost half of the production costs directed to obtaining the starting solar-grade silicon (Si) wafers (which have to meet a certain minimum thickness). Although widely used as a solar material, Si has a long absorption length due to its indirect bandgap. For example, it takes a 100 micrometer (μm)-thick Si wafer to absorb 90 percent (%) of sunlight energy above the 1.12 electron volt (eV) bandgap. See, for example, B. M. Kayes et al., “Comparison of the Device Physics Principles of Planar and Radial p-n Junction Nanorod Solar Cells,” J. Appl. Phys., vol. 97, 114302 (2005) (hereinafter “Kayes”).
Alternatively, lower cost materials, such as metallurgical grade poly or multi-crystalline Si, may be used. For example, in 2007 metallurgical grade Si cost about two dollars per kilogram (kg) while high quality solar-grade Si substrates cost about 20 dollars per kg. See, for example, USGS, Silicon: USGS Mineral Commodity Summaries, January 2008. These lower cost materials, however, typically have a very short minority carrier lifetime (and thus a short diffusion length) due to the presence of many impurities and grain boundaries. This unfavorable characteristic severely limits the cell efficiency.
One way to circumvent this problem is to seek efficiency enhancement in these lower cost materials. A promising approach to efficiency enhancement is to direct the carrier collection in a horizontal rather than vertical direction. This scheme can be realized in a cylindrical p-n junction configuration which allows more efficient carrier collection for short diffusion length materials as the collection occurs in a radial direction. See, for example, Kayes which describes nanorod solar cells. Unfortunately, however, conventional techniques for producing nanoscale solar devices are limited by low throughput and thus can be prohibitively expensive for large-scale commercial implementation.
Therefore, improved solar cell fabrication techniques that lower production costs and increase throughput would be desirable.